Put the pancetta on a baking sheet, leaving a few inches of space between slices, and sprinkle with pepper. Bake until crispy, 10 to 15 minutes. Let cool.
Preheat the broiler. Heat the oil in an 8-inch nonstick pan over medium-high heat. Add the onions and cook until soft, about 5 minutes. Add the garlic and cook 30 seconds longer. Season with salt and pepper.
Whisk the eggs in a large bowl until light and fluffy. Whisk in the goat cheese and herbs. Add the egg mixture to the pan with the onions and garlic, and stir just until soft curds begin to form. Sprinkle with some salt. Transfer the pan to the broiler. Broil until the frittata is lightly golden brown and just set, 3 to 5 minutes. Invert onto a platter and let cool slightly or to room temperature, then cut into 4 pieces to fit the rolls.
Spread some hot pepper relish on the bottoms of the rolls. Top with a piece of frittata, 2 slices of pancetta, some arugula, and the roll tops.